In the field of an optical network that is the basis of an Internet communication network, the optical MEMS (Micro Electro Mechanical Systems) technique is moving into the limelight as a technique of implementing multi-channel, WDM (Wavelength Division Multiplexing), and cost reduction, and an optical switch using the optical MEMS technique has been developed (reference 1: Japanese Patent Laid-Open No. 2003-057575). The most characteristic component of the MEMS optical switch is a micromirror array formed by arraying a plurality of micromirror devices.
An optical switch enables path switching without converting light into an electrical signal. Use of the optical switch also makes it possible to switch the path without demultiplexing multiplexed light into wavelengths. Such an optical switch is used to, e.g., upon failure occurrence in a use path, distribute a signal to another path and maintain a communicable state.
In recent years, research and development of wavelength selective switches is in progress, which demultiplex multiplexed light into wavelengths and individually select the paths of light of the respective wavelengths. These wavelength selective switches also use micromirror devices.
A micromirror device (micromirror array) disclosed in reference 1 will be explained below with reference to FIGS. 29 and 30. The micromirror array includes a mirror substrate and an electrode substrate facing it. The mirror substrate has a plurality of movable structures acting as a mirror, and a support member which rotationally supports the movable structures via spring members such as torsion springs. For the electrode substrate, a plurality of electrode portions corresponding to the movable structures acting as a mirror are formed on a substrate serving as a base.
FIG. 29 is a perspective view schematically showing the arrangements of a mirror substrate and an electrode substrate. FIG. 30 is a sectional view schematically showing the arrangement of a micromirror device. Note that FIGS. 29 and 30 partially illustrate a micromirror device which is mainly one constituent unit of a micromirror array. A micromirror array is formed by one- or two-dimensionally arraying the micromirror devices shown in FIGS. 29 and 30. Each micromirror device includes a mirror substrate 200 having a mirror, and an electrode substrate 300 having electrodes. The mirror substrate 200 and the electrode substrate 300 are arranged in parallel to each other.
The mirror substrate 200 includes a plate-shaped base portion 210, a ring-shaped gimbal 220, and a disc-shaped mirror 230. The base portion 210 has an opening having an almost circular shape viewed from above. The gimbal 220 is arranged in the opening of the base portion 210 and connected to the base portion 210 via a pair of connectors 211a and 211b. The gimbal 220 also has an opening having an almost circular shape viewed from above. The mirror 230 is arranged in the opening of the gimbal 220 and connected to the gimbal 220 via a pair of mirror connectors 221a and 221b. A frame portion 240 is formed around the base portion 210 to surround the gimbal 220 and the mirror 230. The frame portion 240 is fixed to the base portion 210 via an insulating layer 250.
The connectors 211a and 211b are provided in the notches of the gimbal 220. The connectors 211a and 211b are formed from meander-shaped torsion springs and connect the base portion 210 to the gimbal 220. The gimbal 220 thus connected to the base portion 210 can rotate about a rotation axis (gimbal rotation axis) which passes through the connectors 211a and 211b. The mirror connectors 221a and 221b are provided in the notches of the gimbal 220. The mirror connectors 221a and 221b are formed from meander-shaped torsion springs and connect the gimbal 220 to the mirror 230. The mirror 230 thus connected to the gimbal 220 can rotate about a rotation axis (mirror rotation axis) which passes through the mirror connectors 221a and 221b. Note that the gimbal rotation axis and the mirror rotation axis are perpendicular to each other.
On the other hand, the electrode substrate 300 includes a plate-shaped base portion 310, a projecting portion 320 which projects from the upper surface of the base portion 310, and a pair of convex portions 360a and 360b which are formed at the periphery of the projecting portion 320 to be juxtaposed while sandwiching the projecting portion 320. The projecting portion 320 includes a second terrace 322 having a truncated pyramidal shape, a first terrace 321 having a truncated pyramidal shape and formed on the upper surface of the second terrace 322, and a pivot 330 having a truncated pyramidal shape and formed on the upper surface of the first terrace 321. The pivot 330 is arranged in correspondence with the central portion of the mirror 230.
Fan-shaped electrodes 340a, 340b, 340c, and 340d are formed on the upper surface of the electrode substrate 300 including the outer surface of the projecting portion 320 so as to be located in a circle concentric to the mirror 230 of the opposite mirror substrate 200. Electrical interconnections 370 are formed inside the convex portions 360a and 360b at the periphery of the projecting portion 320 on the electrode substrate 300. The electrodes 340a to 340d are connected to the electrical interconnections 370 via leads 341a to 341d. The electrodes and the electrical interconnections are formed on an insulating layer 311 which is formed on the surface of the electrode substrate 300.
In the mirror substrate 200 and the electrode substrate 300 which have the above-described arrangements, the mirror 230 faces the corresponding electrodes 340a to 340d. Additionally, the lower surface of the base portion 210 is bonded to the upper surfaces of the convex portions 360a and 360b of the base portion 310 via the insulating layer 311, thereby forming the micromirror device shown in FIG. 30.
In this micromirror device, the mirror 230 is grounded, and positive or negative voltages are applied to the electrodes 340a to 340d to generate an asymmetrical potential difference between them. This allows to attract the mirror 230 by an electrostatic attraction and make it rotate in an arbitrary direction. When forming, e.g., a 1-input 2-output optical switch using the micromirror device, the tilt angle of the mirror 230 is controlled to irradiate the mirror 230 with an optical signal from the input port and make the light reflected by the mirror 230 incident on one of the two output ports.